Nuclear weapons can be launched from air, sea or land, and SSBN boats are hidden in ocean depths so that they can survive a nuclear attack by a hostile country, and then be able to take retaliatory action.
India has a declared No-First-Use (NFU) nuclear doctrine, which however promises massive punitive destruction in retaliation. Submarine-launched nuclear weapons are part of this strategy. Once INS Arihant is operational in 2015, India will then complete the nuclear triad of air, surface and underwater nuclear attack capability.
Nuclear weapons are under the tri-service Strategic Forces Command (SFC) and top level clearance is required from the government to launch them if ever needed.